Hydrogen order in monoclinic ZrCr2H3.8

The hydrogen distribution in the cubic Laves phase derivative ZrCr2H3.8 has been investigated by X-ray and neutron powder diffraction on the deuteride in the temperature range 1.6 K less than or equal to T less than or equal to 293 K. The crystal structure transforms at 250 K from a disordered cubic high-temperature modification (space group Fd (3) over bar m, a = 769.13(1 ) pin at T = 293 K, Z = 8) to an ordered monoclinic low temperature modific ation (space group C2/c, a = 938.16(1), b = 544.45(1), c = 934.959(1) pm, b eta = 108.946(1)degrees at T = 1.6 K, Z = 8), The latter displays three chr omium sites which are surrounded by four almost fully occupied deuterium si tes in saddle-like, distorted tetrahedral and planar configurations with Cr -D distances in the range 174-183 pm. (C) 1999 Elsevier Science S.A. All ri ghts reserved.